Phase transition of Zn0 under high pressure and temperature

Transcription

1 No. l ] Proc. Japan Acad., 75, Ser. B (1999) 1 Phase transition of Zn0 under high pressure and temperature By Keij i KUSABA, *) Yasuhiko SYONO, **) and Takumi KIKEGAWA ** (Communicated by Syun-iti AKIMOTO, M.J.A., Jan. 12, 1999) 0 O.1MPa Abstract: Pressure induced B4 (wurtzite) - B1(rocksalt) transition of Zn0 was investigated by energy dispersive X-ray diffraction method up to 12GPa and 1000 C. The phase boundary between the B4 and B1 phases can be described by a straight line with P/GPa = *T/ C. Under non-hydrostatic condition, the phase transition at 27 C is reversible with a large hysteresis about ± 4.OGPa, and the high pressure phase with the B1 structure is unquenchable at ambient condition. On the other hand, the B1 type high pressure phase is quenchable under hydrostatic condition. The unit cell parameter of the B1 phase at and 27 C is determined to be a = (3)A. Bulk moduli of the B4 and B1 phases at 27 C are determined to be 128.4±0.9GPa and 197±3GPa, respectively. Furthermore, an anisotropic compression behavior of the B4 type Zn0 is observed, in which the c axis of the B4 type structure is slightly more compressible than that of the a axis. Key words: ZnO; high pressure; phase transition; phase diagram. Introduction. Zinc oxide has the wurtzite (B4) type structure with c/a = at ambient condition. Bates et al. (1962)1 first discovered a high pressure phase with the rocksalt (B1) type structure, volume of which was by about 18% smaller than that of the B4 type phase at ambient condition. They claimed that the B1 phase was quenchable and that the phase boundary had a positive slope based on their quenching experiments. Since the discovery of the B1 phase, there have been many investigations on the high pressure behavior of Zn0 from the viewpoints of materials science, solid state physics and earth science.2~-10~ Due to strong tetrahedral preference, the high pressure behavior of Zn2+ is very different from those of Mgt, Fee, Co2+ and Ni2+.11)-13) Zn0 is also an important material as one of the II-VI group semiconducting materials and piezoelectric transducers. There remained three unsolved problems in the previous investigations. The first problem is a disparity about phase boundary between the B4 and B1 phases. Inoue (1975)4 observed the B4-B1 transition of Zn0 under high pressure and temperature up to 8.5GPa and *) I nstitute for Materials Research, Tohoku University, Katahira, Sendai, Miyagi , Japan. * *) I nstitute for Materials Structure Science, High Energy Accelerator Organization, 1-1 Oho, Tsukuba, Ibaraki , Japan C by his own developed in-situ observation technique, a combination of a large volume cubic press and energy dispersive X-ray diffraction method using a laboratory X-ray source. According to his study, the equilibrium phase boundary can be described by P/GPa = *T/ C. The determined phase boundary has a negative slope and is quite different from the previous report' by quench method, in which the boundary has a positive slope. The second one is concerned with the compression behavior of the B4 type phase. Inoue (1975)4 reported that the [001] direction of the B4 structure is slightly more compressive than the [100] direction. However, Ahuja et al. (1998)9 recently predicted that the [001] direction of the B4 type structure would be harder than the other direction using accurate first-principle total energy calculation method. Unfortunately, Inoue's investigation was published only in his Ph. D. thesis,4 and his remarkable results have never been open to arguments. The third one is about the quenchabllity of the B1 phase to ambient condition. For example, Jamieson (1970)3 reported that the phase transition was reversible at room temperature and pointed out that the quenched B1 phase by Bates et al. (1962)1 was stabilized by NH4Cl. On the other hand, Gerward & Olsen (1995)7 succeeded to quench a large volume fraction of

2 2 K. KUSABA, Y SYONo, and T. KIKEGAWA [Vol. 75(B) the B1 type ZnO using high purity starting material. Such quenchability of the high pressure phase is a very important problem in the field of materials science. In a past quarter century, the Inoue's technique4~ using a laboratory X-ray source evolved to the use of synchrotron X-ray source,14~ and has provided us easy in-situ observation under high pressure and temperature. The aim of the present study is to redetermine equilibrium phase boundary between the B4 and B1 phases using synchrotron X-ray source, and to observe the compression and decompression behavior of ZnO. Our final goal is to consider the quenchability of the B1 type phase to ambient condition. Experiments. In the present study, two kinds of ZnO were used fof the starting material to examine the impurity effect on the quenchability of the B1 phase. One was a chemical reagent of ZnO (5N) provided by Rare Metallic Co., Ltd. It was confirmed to be a single phase with the B4 type structure by X-ray powder diffraction method. The parameters of a hexagonal cell were calculated lated to be a = 3.250A, c = 5.207A and c/a =1.602, and the values were in good agreement with those of ICCD card (a = A and c = A). This starting material was used in 15 runs of 18 high pressure and temperature experiments. The other was synthesized material from high purity zinc metal (4N) for chemical analysis. This starting material was expected to be free from NH4Cl.1~'3~ The material was partially used in the present experiments to check the impurity effect on stabilization of the B1 phase. In-situ X-ray observation of ZnO under high pressure and temperature was carried out by the energy-dispersive type X-ray powder diffraction method using a combination of a large volume cubic press and a strong synchrotron X-ray source. Two high pressure experimental stations (the MAX80 system14~ at AR-NE5C and the MAX9o systeml5> at PF-14C) in Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK) were used in this study. The MAX80 system and the MAX9o system fundamentally have the same DIA-lo type high pressure vessel, which can generate high pressure and temperature conditions of more than 15GPa and 1000 C with 3mm size sintered diamond anvils,15~ and useful energy range for the X-ray diffraction method was up to about lookev. In the present study, the high pressure behavior of ZnO was observed under both hydrostatic and nonhydrostatic compression conditions with and without liquid pressure medium respectively. The outline of Fig. 1. X-ray powder diffraction patterns of reversible B4-B1 transition of Zn0 under non-hydrostatic condition, taken at 29=7.99. a) B4 type phase before compression. Diffraction lines were indexed on a hexagonal cell. b) B1 type phase at 7.77GPa and 550 C. Lines were indexed on the basis of a cubic cell. c) Reverted B4 type phase after decompression process to O.1MPa and 27 C. high pressure-temperature generations and measurements under non-hydrostatic condition was described in our previous report.16~ We attended to reduce strain effects of the sample and used NaCI as a pressure marker17~ under non-hydrostatic condition. Every X-ray pattern under high pressure condition was collected after heating to 400 C or higher. In the case of the hydrostatic experiment, ZnO and NaCI as a pressure marker were sealed in an aluminum alloy (A12024) capsule with a 4:1 methanollethanol mixture, and the capsule was encased in a solid pressure medium for a high pressure and temperature experiment. Even if the aluminum alloy capsule was used, X-ray diffraction lines could be observed in the energy range higher than about 4okeV. Results and discussion. Reversible transition under non-hydrostatic condition. Fig. l shows typical X-ray powder diffraction patterns of ZnO under nonhydrostatic condition. All X-ray diffraction lines before compression could be indexed as the B4 type phase, as shown in Fig. l-a). No extra X-ray diffraction lines were observed in the compression process up to about 8GPa at room temperature. The X-ray pattern was drastically changed by elevating temperature up to 550 C, as

3 No. l ] Phase transition of ZnO 3 shown in Fig. 1-b). Diffraction lines from the B4 type phase completely disappeared and only new diffraction lines which could be assigned as the B 1 type phase were observed. The B 1 phase was stable in the temperature cooling process and the decompression process down to about 3GPa at room temperature. Intensities of the B 1 type diffraction lines became weak and diffraction lines from the B4 type phase appeared around 2GPa. Finally, broadened diffraction lines of the B4 phase were observed with a trace of the B1 phase at ambient condition, as shown in Fig. 1-c). No difference was observed between two kinds of starting materials in stabilization of the B1 phase. This experimental result showed that the B4-B1 transition under non-hydrostatic condition was reversible and that the transition had a large hysteresis in both pressure and temperature cycles. The line broadening of the diffraction lines from the reverted B4 type phase were also observed in ZnO obtained by pressure and temperature induced decomposition of Zn2SiO4 and ZnSiO3.12~ Determination of phase boundary. In order to determine the equilibrium phase boundary, two kinds of experiments were carried out by taking the large hysteresis of the transition into consideration. One was direct determination of the transition pressure for both the B4-B1 transition and its reverse transition at an isothermal condition up to 900 C. In the other type experiment, a mixture of the B4 and B1 phases was kept at the desired pressure and temperature condition for 3 hours, and the change of the ratio between the two phases was observed. Experimental results are summarized in Fig. 2. The difference in the phase transition pressures during loading and unloading at 27 C was as large as 8GPa, but it was decreased by elevating temperature. The hysteresis at temperatures higher than 600 C was smaller than 0.5GPa, and could be ignored for determination of the phase boundary. The large hysteresis at 27 C was consistent with the previous study.8~ No remarkable difference about the temperature dependence of the hysteresis was observed between the two kinds of starting materials. The solid symbol marks in Fig. 2 indicate the stability points determined by relative change of volume fractions of the B4 and B1 phases obtained by holding 3 hours, as shown in Figs. 3 and 4. From the present results, the equilibrium phase boundary between the B4 and B1 phases was described by a straight line with P/GPa = *T/ C, as shown in Fig. 2. The phase transition pressure was by 25% lower than the Fig. 2. High pressure and temperature phase diagram of ZnO. Open circle and square marks indicate the P and T conditions, where single phase of the B4 and B1 type phases were observed, respectively. Solid marks represent the P and T conditions, where the volume fraction ratio between the B4 and Bi phases was changed in 3 hours, as shown in Figs. 3 and 4. Small triangle marks showp and T conditions for B4-B1 or B1-B4 transition under isothermal compression or decompression process. The thin lines show pressure hysteresis of the phase transition. The determined phase boundary is indicated by a thick line. Phase boundaries of Bates et al. (1962)1 and Inoue (1975)4 are shown by dotted line and broken line, respectively. Fig. 3. Change of X-ray diffraction pattern at 5.9GPa and 300 C, as shown by the solid square mark in Fig. 2. a) Initial state. Circle and square marks indicate the diffraction lines of the B4 and B1 phases, respectively. b) After 3 hours. The change in the X-ray diffraction patterns indicates that the B1 phase is stable in this P and T condition. accurate result.9~ first-principles total energy calculation

4 4 K. KUSABA, Y. SYONO, and T. KIKEGAWA [Vol. 75(B) Fig. 4. Change of X-ray diffraction pattern at 5.6GPa and 300 C, as shown by the solid circle mark in Fig. 2. a) Initial state. circle and square marks indicate the diffraction lines of the B4 and B1 phases, respectively. b) After 3 hours. The change in the X-ray diffraction patterns indicates that the B4 phase is stable in this P and T condition. 0 dition The present phase boundary has a negative slope, and it is inconsistent with that by Bates et al. (1962),1) as indicated by a dot line in Fig. 2. The pressure value was corrected because they used the old pressure scale.l),4) The present boundary was fundamentally consistent with the Inoue's result (1975),4) as indicated by a broken line in Fig. 2, although our result slightly shifted to lower pressure side than Inoue's one. The difference might be explained by Inoue's determination method4): He determined the boundary only by observing transition from the B4 phase to B1 phase. Probably, his boundary was influenced by the hysteresis of the transition, particularly in the low temperature region. In fact, the difference in the low temperature region between the present boundary and Inoue' one was larger than that in the high temperature region. Irreversible transition under hydrostatic condition. Fig. 5 shows the X-ray diffraction patterns observed under hydrostatic condition. In this case, the B1 type phase was quenched to ambient condition, although experimental process was almost the same as that under non-hydrostatic condition, as shown in Fig. 1. The cell parameter of the B1 type phase at ambient conwas determined to be a = (3)A from d-values of the X-ray diffraction lines, as listed in Table I. The volume of the B1 phase was calculated to be by 17.8% smaller than that of the B4 type phase. The cell parameter was in good agreement with the previous studies [Bates et al. (1962)1); 4.280A, Liu (1977)13) 4.275A and Gerward & Olsen (1995)7); 4.280A and Fig. 5. X-ray powder diffraction patterns of irreversible B4-B1 transition of ZnO under hydrostatic condition, taken at 29=7.99. a) B4 type phase before compression. b) B1 type phase at 7.55GPa and 400 C. c) B1 type phase quenched to O.1MPa and 27 C. Table I. Observed and calculated d-values of B1 type ZnO at O.1MPa and 27T Recio et al. (1998)10); 4.275A}. The agreement suggests that the B 1 phase quenched to ambient condition was free from impurities in these studies. Compression behavior of ZnO. Compressibilities of both the B4 and B1 phases at 2TC are shown in Fig. 6. Bulk moduli of the B4 and B 1 phases were calculated to be ± 0.86GPa and ± 3.1 GPa, respectively, based on the Birch-Murnaghan equation with K'=4. These values were different from those of previous studies (Gerward & Olsen (1995)7; 136GPa for B4 phase and 17OGPa for B1 phase, and Karzel et al.

5 No. l ] Phase transition of Zn0 5 Fig. 6. Compression behavior of B4 and B1 type Zn0 at 27 C. Fig. 7. Anisotropic compression behavior of B4 type Zn0 with a hexagonal cell at 27 C. (1996)8; 183GPa for B4 phase and 228GPa for B1 phase). The reason for this difference has not been explained yet. Fig. 7 shows the pressure dependence of the c/a ratio of the B4 type phase at 2TC. The c/a ratio was at ambient condition, and slightly smaller than the ideal value (1.633), i. e. the B4 type structure could be considered to be slightly compressed along the c axis even at ambient condition. Fig. 7 indicates that the c axis direction of the B4 type structure was softer than the other directions. The tendency was consistent with Inoue (1975),4 but was opposite to the prediction based on accurate first-principles total energy calculations.9~ Quenchability of B1 type ZnO to ambient condition. We discuss the quenchability of the B1 phase in this section, because quenchability of a high pressure phase is a very important problem for high pressure research, particularly in the field of material science. There has been complicated situation in the previous studies about the high pressure behavior of ZnO: Bates et al. (1962),1 Liu (1977),13 Gerward & Olsen (1995)7 and Recio et al. (1998)10 reported that the B1 phase was quenchable, on the other hand Jamieson (1970),3 Ito & Matsui (1974)12 and Karzel et al. (1996)8) claimed that the B1 phase was unquenchable. In the beginning of investigations, the high pressure phase with the B1 type structure was considered to be quenchable with the existence impurities for example NH4Cl.1~'3~ However, the hypothesis could not explain Gerward & Olsen (1995),7 in which starting material was expected to be free from impurities. Also, the present result using high purity starting material is also negative against the hypothesis. We propose a new assumption: Crystallinity of the Bl phase produced at high perssure will play an important role in the quenchability, i.e. well-crystallized B1 type phase may be quenchable. Long holding interval, over pressure or catalysis may lead to well crystallization of the B1 phase under high pressure condition. The assumption can explain the different results of room temperature experiments between Gerward & Olsen (1995)7 and Karzel et al. (1996)8; ZnO was compressed up to 3OGPa and B1 phase was quenchable in Gerward & Olsen (1995),7 on the other hand Karzel et al. (1996)8) compressed to only 1OGPa and it was unquenchable. In the case of Bates et al. (1962),1 the B1 phase was quenchable under relatively high pressure condition. NH4C1 might play a catalysis effect. The authors also tried to increase the yield of the B1 phase by long holding time of the high pressure experiments. In the present study under hydrostatic compression, pressure medium of a 4:1 methanollethanol mixture may help crystallization of the B1 phase at high pressure and temperature. Acknowledgements. The authors wish to thank Dr. Katsuhiko Inoue, Kobe Steel, Ltd., and Prof. Syun-iti Akimoto, M. J. A., for their helpful comments and discussions. This study has been performed under the approval of the Photon Factory Program Advisory Committee (Proposal No. 98G259). 1) 2) 3) 4) References Bates, C. H., White, W. B., and Roy, R. (1962) Science 137, 993. Bateman, T. B. (1962) J. Appl. Phys. 33, Jamieson, J. C. (1970) Phys. Earth Planet. Interiors 3, Inoue, K. (1975) Ph. D. Thesis, University of Tokyo, Tokyo, pp

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